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CHAPTER XIII

STATIC ELECTRICITY

GENERAL FACTS OF ELECTRIFICATION

278. Electrification by friction. If a piece of hard rubber or a stick of sealing wax is rubbed with flannel or cat's fur and then brought near some dry pith balls, bits of paper, or other light bodies, these bodies are found to jump toward the rod. This sort of attraction, so familiar to us from the behavior of our hair in winter when we comb it with a rubber comb, was observed as early as 600 B.C., when Thales of Greece commented on the fact that rubbed amber draws to itself threads and other light objects. But it was not until A.D. 1600 that Dr. William Gilbert, physician to Queen Elizabeth, and sometimes called the father of the modern science of electricity and magnetism, discovered that the effect could be produced by rubbing together a great variety of other substances besides amber and silk, such, for example, as glass and silk, sealing wax and flannel, hard rubber and cat's fur, etc.

The effect produced upon these various substances by friction was named by Gilbert (see opposite page 238) electrification, after the Greek name electron, meaning "amber." Thus a body which, like rubbed amber, has been endowed with the property of attracting light bodies is said to have been electrified, or to have been given a charge of electricity. In this statement nothing is said about the nature of electricity. We simply define an electrically charged body as one that has been put into the condition in which it acts toward light bodies like the rubbed amber or the rubbed sealing wax. We do not know with certainty what the ultimate nature of

electricity is, but we are fairly sure of the laws that govern its action. The following sections deal with these laws.

279. Positive and negative electricity. Let a pith ball suspended by a silk thread, as in Fig. 221, be touched to a glass rod that has been rubbed with silk; the ball will thus be put into the condition in which it is strongly repelled by this rod. Next let a stick of sealing wax or an ebonite rod

that has been rubbed with cat's fur or flannel be brought near the charged ball. It will be found that it is not repelled but, on the contrary, is very strongly attracted. Similarly, if the pith ball has touched the sealing wax so that it is repelled by it, it is found to be strongly attracted by the glass rod.

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FIG. 221. Pith-ball electroscope

Again, two pith balls both of which have been in contact with the glass rod are found to repel each other, but pith balls one of which has been in contact with the glass rod and the other with the sealing wax attract each other.

Evidently, then, the electrifications imparted to glass by rubbing it with silk and to sealing wax by rubbing it with flannel are opposite in the sense that an electrified body which is attracted by one is repelled by the other. We say, therefore, that there are two kinds of electrification. Benjamin Franklin arbitrarily introduced the terms positive and negative, or + and -, to designate these two kinds of electrification. Thus, a positively electrified body is one which acts with respect to other electrified bodies like a glass rod that has been rubbed with silk, and a negatively electrified body is one which acts like a piece of sealing wax that has been rubbed with flannel. These facts and definitions may be stated in the following general law: Electrical charges of like kind repel each other, and charges of unlike kind attract each other. The forces of attraction or repulsion are found, like those of gravitation and magnetism, to decrease as the square of the distance increases.

280. Measurement of electrical quantities. The fact of attraction and repulsion is taken as the basis for the definition and measurement of so-called quantities of electricity. Thus, a small charged body is said to contain 1 unit of electricity when it will repel with a force of 1 dyne an exactly equal and similar charge placed 1 cm. away. The number of units of electricity on any charged body is then measured by the force that it exerts upon a unit charge placed at a given distance from it; for example, a charge that at a distance of 10 cm. repels a unit charge with a force of 1 dyne contains 100 units of electricity, for this means that at a distance of 1 cm. it would repel the unit charge with a force of 100 dynes (see § 279).

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281. Conductors and nonconductors. Let an electroscope E (Fig. 222), consisting of a pair of gold leaves a and b, suspended from an insulated metal rod r and protected from air currents by a case J, be connected with the metal ball B by means of a wire. Now let an ebonite rod be electrified and rubbed over B. The gold leaves will immediately diverge, showing that a portion of the electrical charge placed upon B has been carried by the wire to the gold leaves, where it makes them diverge in accordance with the law that bodies charged with the same kind of electricity repel each other.

FIG. 222. Illustrating conduction

Let the experiment be repeated when E and B are connected with a thread of silk or a long rod of wood instead of the metal wire. No divergence of the leaves will be observed. If a moistened thread connects E and B, the leaves will be seen to diverge slowly when the ball B is charged, showing that a charge is carried slowly by the moist thread.

These experiments make it clear that while electrical charges pass with perfect readiness from one point to another in a wire, they are quite unable to pass along dry silk or wood, and pass with difficulty along moist silk. We are therefore accustomed to divide substances into two classes, con

ductors and nonconductors (or insulators), according to their ability to transmit electrical charges from point to point. Thus, metals and solutions of salts and acids in water are all conductors of electricity, and glass, porcelain, rubber, mica, shellac, wood, silk, vaseline, turpentine, paraffin, and oils are insulators. No hard-and-fast line, however, can be drawn between conductors and nonconductors, since all so-called insulators conduct to some slight extent, and the so-called conductors differ greatly in the facility with which they transmit charges.

The fact of conduction brings out sharply one of the most essential distinctions between electricity and magnetism. Magnetic poles exist only in iron and steel; electrical charges may be communicated to any body whatever, provided it is insulated. These charges pass over conductors and can be transferred by contact from

one body to any other, whereas magnetic poles remain fixed in position and are wholly uninfluenced by contact with other bodies unless these bodies are themselves magnets.

282. Electrostatic induction. Let the ebonite rod be electrified by friction and slowly brought toward the knob of the gold-leaf

FIG. 223. Illustrating induction

electroscope (Fig. 223). The leaves will be seen to diverge, even though the rod does not approach to within a foot of the electroscope.

This makes it clear that the mere influence which an electrical charge exerts upon a conductor placed in its neighborhood is able to produce electrification in that conductor. This method of producing electrification is called electrostatic induction.

As soon as the charged rod is removed, the leaves will be seen to collapse completely. This shows that this form

of electrification is only a temporary phenomenon which is due simply to the presence of the charged body in the neighborhood.

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283. Nature of electrification produced by induction. Let a metal ball A (Fig. 224) be strongly charged by rubbing it with a charged rod, and let it then be brought near an insulated * metal body B provided with pith balls or strips of paper a, b, c, as shown. The divergence of a and c will show that the ends of B have received electrical charges because of the presence of A, and the failure of b to diverge will show that the middle of B is uncharged. Furthermore, the rod that charged A will be found to repel c but to attract a.

FIG. 224. Nature of induced charges

We conclude, therefore, that when a conductor is brought near a charged body, the end away from the inducing charge is electrified with the same kind of electricity as that on the inducing body, while the end toward the inducing body receives electricity of the opposite kind.

284. The electron theory of electricity. The atoms of all substances are now known to contain as constituents both positive and negative electricity, the latter existing in the form of minute corpuscles, or electrons, each of which has a mass 145 of that of the hydrogen atom. These electrons are grouped in some way about the positive electricity as a nucleus. The sum of the negative charges of these electrons is equal to the positive charge of the nucleus, so that in its normal condition the whole atom is neutral, or uncharged. But in conductors electrons are continually getting loose from the atoms and reëntering other atoms, so that at any given instant there are in every conductor a number of free negative electrons and a corresponding number of atoms which have lost electrons and which are therefore positively charged.

* Sulphur is practically a perfect insulator in all weathers, wet or dry. Metal conductors of almost any shape resting upon pieces of sulphur will serve the purposes of this experiment in summer or winter.

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